Due to restrictive patents, new potentially life-saving HIV drugs are "beyond reach", Doctors Without Borders (MSF) said even though basic medication for the disease has become cheaper.
The international medical humanitarian organisation said it was good news that the price of drugs used as first- and second-line treatments had fallen by 19 and 28 percent respectively since last year.
But "salvage regimens" -- drugs used as a last resort after these first two steps fail -- cost 15 times as much as first-line medicines, according to an MSF statement at the International AIDS Society conference in the Malaysian capital.
"Patents keep them priced beyond reach. We need to watch carefully as newer, better medicines reach the market in the coming years, as these are the drugs that we'll quickly be needing to roll out. The price question is far from resolved," said Jennifer Cohn, medical director at MSF's Access Campaign.
According to MSF, first-line treatment -- which has the highest efficacy against a low side-effect profile -- can cost as little as $139 per person per year, down from $172.
It cited generic competition as the main factor in the price drop, which had also made second-line drugs available for as low as $303 per year.
But MSF said its research found the best price for a possible salvage regimen was $2,006 per year in the poorest countries, while some like Armenia pay $13,213 just for one of the three or four drugs needed for a full regimen.
"Scaling up HIV treatment and sustaining people on treatment for life will depend on bringing the price of newer drugs down," said Arax Bozadjian, HIV pharmacist at MSF's Access Campaign.
The group called for wider "patent opposition", claiming that a seven-year battle against Swiss pharmaceutical firm Novartis, was key to falling prices of first- and second-line regimens. Novartis were finally refused a patent this April in generics-producing India.
"When patents prevent access, compulsory licences should be issued (to other drugmakers) in the interest of public health," it added.
The United Nations says 34 million people globally were living with HIV in 2011, with 2.5 million people newly infected that year alone.
In total, the global pandemic has claimed 30 million lives.